The present invention relates to a method for controlling a flexible line comprising at least a layer of armors surrounded by an external sheath, the external sheath delimiting an inner space receiving the layer of armors and comprising at least one medium at the interface between the external sheath and the internal space, the method comprising:                the sending of an ultrasonic signal from the outside of the external sheath onto a region to be controlled of the external sheath, and        the reception of the signal reflected at the interface between the region to be controlled of the external sheath and the inner space facing the region to be controlled of the external sheath.        
The invention also relates to an installation for controlling a flexible line.
In particular, the method is intended for detecting flooding of the space present inside the external sheath, notably flooding of the layers of traction armors present in the flexible line.
The flexible line is advantageously a flexible conduit of the unbonded type intended for the transport of hydrocarbons through an extent of water, such as an ocean, a sea, a lake or a river. Alternatively, the flexible line is a reinforced umbilical line with elements of armor or further a cable.
Such a flexible conduit is for example made according to the normative documents API 17J (Specification for Unbonded Flexible Pipe) and API RP 17B (Recommended Practice for Flexible Pipe) established by the American Petroleum Institute.
The conduit is generally formed with a set of concentric and superposed layers. It is considered as “unbonded” in the sense of the present invention from the moment that at least one of the layers of the conduit is able to be moved longitudinally relatively to the adjacent layers during flexure of the conduit. In particular, an unbonded conduit is a conduit without any binding materials connecting layers forming the conduit.
The conduit is generally positioned through an extent of water, between a bottom assembly, intended to collect the fluid exploited in the bottom of the extent of water and a fixed or floating surface assembly, intended to collect and distribute the fluid. The surface assembly may be a semi-submersible platform, an FPSO or another floating assembly.
The conduits intended for great depths also have to resist to very strong tensions, currently several tens of tons, to which they are subject during operation and/or during their installation in the sea.
Further, in the case when the surface assembly is floating and mobile depending on the sea conditions, the riser conduits (“risers”) connecting the sea bottom to the surface assembly may sometimes be subject to millions of curvature variation cycles. These riser conduits therefore should also be capable of resisting durably to dynamic fatigue stresses.
In order to guarantee such a strength in tension during the whole lifetime of the conduit, it is necessary to ensure the integrity of the layers of traction armors, generally made from helical windings of metal wires. In particular, the layers of armors are in certain cases sensitive to corrosion. The corrosion may be induced by the permeation of acid compounds present in the transported fluid and/or by the presence of water in the space receiving the layers of armors.
The presence of water may notably appear during a flaw or breakage of the external sheath which then no longer ensures its protective function of the conduit.
In order to detect possible flaws or breakages of the external sheath leading to flooding of the internal space, different tests are applied, such as the annular test. The annular test consists of measuring the current volume of gas of the annular portion of the flexible conduit for example by applying vacuum in the annular portion. The measured current volume of gas is compared with the initial volume of the annular portion so as to infer therefrom whether water has invaded partly or totally the annular portion. However, such a volume measurement is often not very accurate and therefore does not give the possibility of determining the presence and the height of possible flooded areas threatening the integrity of the flexible conduit.
GB-B-2 446 670 describes an underwater inspection method for the integrity of the annular portion of a flexible conduit based on the ultrasonic echography technique. According to this method, an ultrasonic probe emits an ultrasonic wave which penetrates into the conduit. In return, the probe receives the ultrasonic waves reflected at the discontinuities, i.e. at the interfaces, encountered in the conduit. The amplitudes of the reflected waves notably give the possibility of determining whether the portion of the inspected conduit is flooded.
Such a method is based on a property of ultrasonic waves according to which the ultrasonic waves do not propagate very much in a gas as opposed to a liquid medium such as water.
However, such a property is not verified in a medium subject to external pressure. Indeed from a certain pressure level between two materials, the ultrasonic waves propagate in the absence of a coupling medium such as a liquid. For example, from a few tens of bars of contact pressure, ultrasonic waves propagate between a thermoplastic sheath and a metal layer without any coupling medium.
The coupling pressure between the external sheath and the elements of armors mainly depend on the internal pressure of the flexible conduit and on the hydrostatic pressure. The coupling pressure therefore varies according to the depth at which the measurement is conducted.
Thus, when the contact pressure between the external sheath and the elements of armors is greater than a few tens of bars, the inspection method shown in patent GB-B-2 446 670 does not give the possibility of making a distinction between a flooded annular portion and a dry annular portion.
The object of the invention is to provide a method for controlling the integrity of a flexible conduit, in particular of the annular portion of the flexible conduit, which is not intrusive, simple to apply and reliable regardless of the outer pressure applied to the flexible conduit.
For this purpose, the object of the invention is a method of the aforementioned type, further comprising the analysis of the polarity of the signal reflected at the interface and the determination, according to the analyzed polarity, of at least the nature of the medium at the interface.
The method according to the invention may comprise one or several of the following characteristics, taken individually or according to any technically possible combination:                during the analysis step, the reflected signal is not rectified.        the method further includes the sweeping of a plurality of regions to be controlled successively of the external sheath and the repetition, for each region to be controlled of the external sheath, of steps for sending, receiving, analyzing and determining the medium at the interface between the region to be controlled of the external sheath and the internal space facing the region to be controlled of the external sheath.        the sweeping pitch is comprised between 0 millimeter and 10 millimeters and advantageously between 0 mm and 2 mm.        the sweeping is carried out axially along the flexible line and/or angularly around the flexible line.        the sweeping is carried out by using a probe sending the ultrasonic signal borne by a mobile displacement support of the probe.        the mobile support is a motor-driven manipulator attached around the flexible line by means of a underwater robot of the conduit, a mobile manipulator capable of moving along the conduit and/or a mobile support able to be grasped by the hand of a diver.        the sent ultrasonic signal has a central frequency selected from between 1.5 MHz and 5 MHz, preferentially between 2 MHz and 2.5 MHz.        the ultrasonic signal is sent in the form of a focussed ultrasonic beam.        the ultrasonic signal is sent and received by a piezo-electric sensor.        during the determination step, the reflected signal is compared with a database of ultrasonic signals.        the flexible line includes an intermediate layer between the external sheath and the layer of armors.        the medium is a liquid, a gas or a solid.        the method is applied during the use of the flexible line, in particular during the passage of a fluid through a central passage of the flexible line.        the external sheath is immersed in an extent of water, the sending of the ultrasonic signal being carried out by means of a probe oriented towards the external surface of the external sheath.        
The object of the invention is also an installation for controlling a flexible line comprising at least one layer of armors surrounded with an external sheath, the external sheath delimiting an internal space receiving the layer of armors and comprising at least one medium at the interface between the external sheath and the internal space, the installation comprising:                a probe able to send an ultrasonic signal on a region to be controlled of the external sheath and of receiving the reflected signal at the interface between the region to be controlled of the external sheath and the internal space facing the region to be controlled of the external sheath, and        a data processing unit configured for analyzing the polarity of the signal reflected at the interface and determining, according to the analyzed polarity, at least the nature of the medium at the interface.        